Minor bug fixes.

- Bug fix for step and direction invert masks not immediately being in
effect. Now regenerates the masks when a user changes this setting.

- Bug fix for probing cycle. G-code standard mandates that there is an
error if the probe is already triggered when the cycle is commanded.
However, Grbl may have motions to pull off a previous probing cycle in
queue and can falsely lead to errors. To fix this, the triggered check
is performed within the probing cycle itself, right after the planner
buffer is synced. If there is an error, it will now alarm out as a
probe fail.

stepper.h

@@ -35,6 +35,9 @@ void st_wake_up();
 // Immediately disables steppers
 void st_go_idle();
 
+// Generate the step and direction port invert masks.
+void st_generate_step_dir_invert_masks();
+
 // Reset the stepper subsystem variables       
 void st_reset();